- Nincada → Ninjask + Shedinja 
Catch a Nincada. Clear up at least one Pokémon slot. Then make your Nincada evolve (about level 20) into a Ninjask. You will then have a Shedinja in the empty slot. The Shedinja will be exactly like the Ninjask, except for the HP.
See also: How do I catch Ninjask in Pokémon